Carcinomatous encephalitis is a rapidly fatal form of metastasis caused by miliary spread of systemic cancer into the brain parenchyma. The diagnostic criteria and optimal treatment for this disease are not well defined. We report a patient with rapid neurologic deterioration from carcinomatous encephalitis from lung adenocarcinoma. She was treated with gefitinib and high-dose fractionated whole brain radiotherapy, and eventually improved neurologically and was discharged home on hospital day 48. Gefitinib and high-dose fractionated radiotherapy may have synergistic activity in patients with carcinomatous encephalitis from non-small cell lung cancer having favorable prognostic factors. More importantly, timely recognition of this disease and the use of large fraction radiation therapy are necessary to control rapid neurologic deterioration.